Friday, April 28, 2017

A new NASA study (published in Journal of Geophysical Research -- Oceans) is challenging a long-held theory that tsunamis form
and acquire their energy mostly from vertical movement of the seafloor.
An undisputed fact was that most tsunamis result from a massive
shifting of the seafloor -- usually from the subduction, or sliding, of
one tectonic plate under another during an earthquake. Experiments
conducted in wave tanks in the 1970s demonstrated that vertical uplift
of the tank bottom could generate tsunami-like waves.
In the following
decade, Japanese scientists simulated horizontal seafloor displacements
in a wave tank and observed that the resulting energy was negligible.
This led to the current widely held view that vertical movement of the
seafloor is the primary factor in tsunami generation.

The animation shows how waves of energy from the Tohoku-Oki earthquake and tsunami of March 11, 2011, pierced through into Earth's upper atmosphere in the vicinity of Japan, disturbing the density of electrons in the ionosphere. These disturbances were monitored by tracking GPS signals between satellites and ground receivers.A model of ocean tsunami wavefronts [Song, 2007] is overlaid in blue to show the correlation between variations in the ionosphere above and ocean surface below.

Note that traveling ionospheric disturbances (TIDs), visible throughout the animation, are correlated with the position of the tsunami.

In 2007, Tony Song, an oceanographer at NASA’s Jet Propulsion
Laboratory in Pasadena, California, cast doubt on that theory after
analyzing the powerful 2004 Sumatra earthquake in the Indian Ocean.
Seismograph and GPS data showed that the vertical uplift of the seafloor
did not produce enough energy to create a tsunami that powerful.
But
formulations by Song and his colleagues showed that once energy from the
horizontal movement of the seafloor was factored in, all of the
tsunami’s energy was accounted for.
Those results matched tsunami data
collected from a trio of satellites –the NASA/Centre National d’Etudes
Spatiales (CNES) Jason, the U.S. Navy’s Geosat Follow-on and the
European Space Agency’s Environmental Satellite.
Further research by Song on the 2004 Sumatra earthquake, using
satellite data from the NASA/German Aerospace Center Gravity Recovery
and Climate Experiment (GRACE) mission, also backed up his claim that
the amount of energy created by the vertical uplift of the seafloor
alone was insufficient for a tsunami of that size.
“I had all this evidence that contradicted the conventional theory, but I needed more proof,” Song said.

His search for more proof rested on physics -- namely, the fact that
horizontal seafloor movement creates kinetic energy, which is
proportional to the depth of the ocean and the speed of the seafloor's
movement.
After critically evaluating the wave tank experiments of the
1980s, Song found that the tanks used did not accurately represent
either of these two variables.
They were too shallow to reproduce the
actual ratio between ocean depth and seafloor movement that exists in a
tsunami, and the wall in the tank that simulated the horizontal seafloor
movement moved too slowly to replicate the actual speed at which a
tectonic plate moves during an earthquake.
“I began to consider that those two misrepresentations were
responsible for the long-accepted but misleading conclusion that
horizontal movement produces only a small amount of kinetic energy,”
Song said.

Building a Better Wave Tank

To put his theory to the test, Song and researchers from Oregon State
University in Corvallis simulated the 2004 Sumatra and 2011 Tohoku
earthquakes at the university’s Wave Research Laboratory by using both
directly measured and satellite observations as reference.
Like the
experiments of the 1980s, they mimicked horizontal land displacement in
two different tanks by moving a vertical wall in the tank against water,
but they used a piston-powered wave maker capable of generating faster
speeds.
They also better accounted for the ratio of how deep the water
is to the amount of horizontal displacement in actual tsunamis.

The new experiments illustrated that horizontal seafloor displacement
contributed more than half the energy that generated the 2004 and 2011
tsunamis.
“From this study, we’ve demonstrated that we need to look at not only
the vertical but also the horizontal movement of the seafloor to derive
the total energy transferred to the ocean and predict a tsunami,” said
Solomon Yim, a professor of civil and construction engineering at Oregon
State University and a co-author on the study.

Photo taken March 11, 2011, by Sadatsugu Tomizawa
and released via Jiji Press on March 21, 2011, showing tsunami waves
hitting the coast of Minamisoma in Fukushima prefecture, Japan.

Credits: Sadatsugu Tomizawa CC BY-NC-ND 2.0

The finding further validates an approach developed by Song and his colleagues that uses GPS technology to detect a tsunami’s size and strength for early warnings.
The JPL-managed Global Differential Global Positioning System (GDGPS)
is a very accurate real-time GPS processing system that can measure
seafloor movement during an earthquake.
As the land shifts, ground
receiver stations nearer to the epicenter also shift.
The stations can
detect their movement every second through real-time communication with a
constellation of satellites to estimate the amount and direction of
horizontal and vertical land displacement that took place in the ocean.
They developed computer models to incorporate that data with ocean floor
topography and other information to calculate the size and direction of
a tsunami.
“By identifying the important role of the horizontal motion of the
seafloor, our GPS approach directly estimates the energy transferred by
an earthquake to the ocean,” Song said.
“Our goal is to detect a
tsunami’s size before it even forms, for early warnings.”